During the past decade, considerable work has been done on producing diamond by chemical vapor deposition (CVD) processes. Kamo et al., "Diamond Synthesis From Gas Phase in Microwave Plasma," Journal of Crystal Growth 62, pp. 642-644 (1983); and Spear, "Growth of Crystalline Diamond from Low-Pressure Gases," Earth and Mineral Sciences, Vol. 56, No. 4 pp. 53-59 (Summer 1987) are representative of such work. Further, U.S. Pat. No. 4,707,384 (Schachner et al.) lists additional references to the substantial work of different parties which has been performed in this general area.
The Kamo et al. and Spear references, cited above, as well as U.S. Pat. No. 4,434,188 (Kamo et al.), disclose the use of microwave energy to create plasma for such a CVD diamond-forming method. (Plasma is ionized gas.)
Although the ground work has been done to show the feasibility of producing diamond by such a CVD process, much remains to be done to improve the economics of the process for commercialization. One of the ways to improve the economic practicabilities of the CVD process is to improve the yield of the process, that is, the total mass of diamond produced from a given quantity of raw materials and power. It is known that the largest diamond growth rates are obtained using plasma deposition methods. In accordance with these methods, a gas mixture containing carbon, hydrogen, and oxygen atoms is partially ionized to create a plasma. In ionizing the gas mixture, some of the carbon atoms produced by the ionization process dissociate from the gas molecules to form diamond crystals on a substrate such as molybdenum or silicon that is near or in the plasma and at a temperature of about 900.degree. C. to 1000.degree. C. Diamond consists of only carbon atoms arranged in a particular tetragonal crystalline structure. As the diamond crystals grow or spread, they become a diamond film. In order to achieve the best yield, and to keep power consumption to a minimum, the hydrogen and carbon atoms produced must be used efficiently to create the diamond. Any regions of the substrate that are outside of the allowable temperature range reduce the efficiency because incident carbon atoms in such regions may form other carbon structures, such as graphite, or combine with other atoms, e.g., hydrogen or oxygen atoms, to produce other molecules rather than a diamond structure. Hence, the energy supplied to dissociate the carbon atoms is wasted, and the yield of the diamond production process correspondingly decreases. What is needed, therefore, is a diamond production process and/or device wherein the relative number of dissociated carbon atoms deposited as diamond is increased.
Further, as is known in the art, the hydrogen atoms present in the partially ionized gas are needed to prevent the formation of graphite on the substrate surface and/or to scavenge any graphite that is formed. Graphite, like diamond, consists only of carbon atoms, but the atoms are arranged in layers with hexagonal symmetry. Such ionized hydrogen atoms disadvantageously recombine on a cold substrate to form hydrogen molecules, thereby losing their effectiveness at inhibiting the formation of graphite, and further wasting their dissociation energy.
Unfortunately, in prior art processes and apparatus, it has been difficult to maintain a uniform substrate temperature because of variations in the plasma temperature and/or distribution near the substrate surface. Plasma temperature variations may produce hot and cold spots on the substrate, reducing the yield of the process for the reasons ascribed above. Plasma distribution variations may likewise reduce the overall efficiency of the production system because the plasma conditions are not optimized for the greatest diamond production, i.e., the needed ionized atoms are not everywhere present at each position on the substrate. Thus, what is needed to improve the yield of the system is a mechanism for producing a uniform plasma, both in terms of temperature and distribution, along the entire length of the substrate on which diamond is deposited.
Unfortunately, producing and maintaining a reasonably uniform plasma over the entire surface area of a substrate at the gas pressures of 4 torr to 10 torr needed for the diamond CVD process is difficult to achieve. For example, in the case of microwave creation of the plasma at a frequency of 2.45 GHz, difficulties arise because the absorption depth for the microwave radiation in the plasma is at most a few centimeters. Such a shallow absorption depth makes maintaining a constant temperature a formidable task. Moreover, if microwave reflections occur, standing waves are set up that have deep minima or nulls spaced one-half wavelength apart. At each of these minima or nulls the plasma is not heated by the microwave energy to the same degree as it is at other locations. Thus, these nulls can significantly limit the effective length of the substrate that can be used for the deposition process. For example, the free-space wavelength at 2.45 GHz is 12.2 cm, yet guide wavelengths of perhaps a few times this length can be achieved in a practical system. However, because of the maxima and minima associated with a standing wave, only some of this overall length could contribute to the production of diamond.
From the above considerations, it is apparent that what is needed to grow diamond crystal or diamond film efficiently with a high yield is a large substrate area over which a uniform plasma is maintained. The degree of uniformity required is such that the substrate temperature remain within the allowable range over the plasma-covered area and that the plasma conditions remain at or close to optimum over this same area. With such a uniform plasma distribution, a uniform nucleation density and growth rate of the diamond crystals or film over the entire substrate area would result.